"Steve Barker" writes:
Since the inlet and outlet are at the top of the tank, and there would be no
inlet for air, how would a leak "anywhere" drain the tank?

The inlet is a dip tube that actually extends most of the way to the
bottom of the tank, so a water leak in the cold-water side of the house
plumbing plus an air leak higher up in the hot-water plumbing could
siphon almost all the water out of the tank.

And, of course, the tank itself (or its drain valve) could spring a
leak.

Dave